1. Field
This disclosure relates generally to non-volatile memories, and more specifically, to soft programming of a non-volatile memory block.
2. Related Art
During a typical erase operation of a non-volatile memory block (e.g., Flash or EEPROM), a pre-program is performed to raise the threshold voltages of the bits of the memory block to a level at or above a program verify voltage. For the erase operation, the pre-program is followed by a Fowler-Nordheim (FN) erase to lower the threshold voltages of the bits of the memory block to a level at or below an erase verify voltage. However, during the FN erase, the resulting distribution may include bits cells which have been overly erased, which results in increased column leakage. Furthermore, this problem of column leakage increases as the bitcells are further scaled, causing, for example, a subsequent program operation to fail due to lowered drain bias, or a read operation to fail since the over-erased bits may prevent sense amplifiers from distinguishing between an erased and a programmed bit. Therefore, a need exists to soft program or compress the distribution of the erased cells so as to reduce the column leakage. Furthermore, when bitcells become smaller, the total erase time is dominated by the soft program.